Spectacular black smokers along the mid-ocean-ridge crest represent a small fraction of total hydrothermal heat loss from ocean lithosphere. Previous models of measured heat flow suggest that 40%–50% of oceanic hydrothermal heat and fluid flux is from young seafloor (0.1–5 Ma) on mid-ocean-ridge flanks. Despite evidence that ridge-flank hydrothermal flux affects crustal properties, ocean chemistry, and the deep-sea biosphere, few ridge-flank vent sites have been discovered. We describe the first known seafloor expressions of hydrothermal discharge from tectonically formed abyssal hills flanking a fast-spreading ridge. Seafloor manifestations of fluid venting from two young East Pacific Rise abyssal hills (0.1 Ma at 10°20′N, 103°33.2′W; 0.5 Ma at 9°27′N, 104°32.3′W) include fault-scarp hydrothermal mineralization and macrofauna; fault-scarp flocculations containing hyperthermophilic microbes; and hilltop sediment mounds and craters possibly created by fluid expulsion. These visible features can be exploited for hydrothermal exploration of the vast abyssal hill terrain flanking the mid-ocean ridge and for access to the subseafloor biosphere. Petrologic evidence suggests that abyssal hills undergo repeated episodes of transitory fluid discharge, possibly linked to seismic events, and that fluid exit temperatures can be briefly high enough to transport copper (≥250 °C).

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